Some months ago I got one of those cheap switch PSU from Aliexpress. Now I started to work with it and befor plugging in to my electronic device (which requires 12v) i noticed i could not get lower than 21v (glad i checked).

I though, ok, i got a wrong labelled unit, but after opening it I saw a big label on a (coil? internal transformer?) with a 12V 30A text.

This opens two possibilities:

  1. The labeling is still wrong and this is the culprit.
  2. Something else is failing.

I saw some big caps and took pictures, perhaps some engineer or hobbyist will be able to tell me if those caps would work on this unit (to rule out mis-labeling) and if the unit is consistent with a 12v one, perhaps someone can help me out in what to look/measure?


Specs Check the yellow label. Caps More caps

  • 6
    \$\begingroup\$ 3. You measured the voltage without a load large enough to keep it in regulation. \$\endgroup\$ Commented Oct 13, 2015 at 0:02
  • 3
    \$\begingroup\$ Many switching power supplies like that require a minium load for corrrect operation. Consult the supply's datasheet to see if there is any mention of a minimum load requirement. \$\endgroup\$ Commented Oct 13, 2015 at 0:16
  • 1
    \$\begingroup\$ Try it with a small load. Spehro may be right but alternatively it may take only a few mA (or tens or hundreds of mA) to bring it within regulation. \$\endgroup\$
    – user16324
    Commented Oct 13, 2015 at 11:05
  • \$\begingroup\$ Thank you guys! Today when i get to work i'll plug a lamp or something an check how it goes. I found a spec sheet online for my model and says "1.All parameters NOT specially mentioned are measured at 230VAC input, rated load and 25℃ of ambient temperature" \$\endgroup\$
    – Gabriel
    Commented Oct 13, 2015 at 16:15

1 Answer 1


Looks to me like something is broken- probably something in the feedback section (low voltage side to the optoisolator, or possibly just on the other side of the optoisolator). Contrary to other opinions, my experience is that these types of power supplies (unlike PC power supplies and some DC-DC converters) should work fine down to zero load. Here, for example, is a so-called 'user manual' from a similar product that specifies it will work down to 0A.

It could be as simple as a bad solder joint (open connection), so you could look for that.

That said, my initial impression is that this thing is pretty crappy and I would never connect it to anything important or where isolation failure might cause risk to human life or property, or where EMI was a factor. With all that metal it's unlikely to directly cause a fire but could smoke a lot. I doubt the 'FCC' and 'ISO' stickers refer to any real testing or processes- there are no file numbers and no manufacturer legal name that I can see. There is obviously no overvoltage protection (OVP) on the output or it would have shut down- that would be considered a mandatory feature in most cases.

Also, note that they did not bother to install the heatsink HS1 on the diode bridge. Probably works fine at 230VAC in China, but may burn up on a North American power supply at full current (especially in high ambient).

  • 3
    \$\begingroup\$ I've used a few (tens) aluminum box cheap chinese PSUs (same voltage~current range) and they all regulate well enough at zero load to not be out of spec voltage wise, so I can corroborate your answer. \$\endgroup\$
    – Wesley Lee
    Commented Oct 13, 2015 at 1:40
  • \$\begingroup\$ So the pot changes the Amperage, not the Voltage? If so, my tester only measures up to 10A :( Dont want to risk of measuring over that (the PSU is rated up to 30A). But as mentioned in a comment upstream, i found a spec sheet that says the numbers are based with load. p.globalsources.com/IMAGES/PDT/SPEC/260/K1079137260.pdf Regarding safety and such, yes, well, its a 10-20 bucks PSU for a 3d Printer. If working under load does not work, i'll completely remove the board and check for solders as recommended. \$\endgroup\$
    – Gabriel
    Commented Oct 13, 2015 at 16:17

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